It is advantageous to heat certain workpieces to a temperature gradient along a dimension of the workpiece. For example a cylindrical aluminum workpiece, or billet, that undergoes an extrusion process is generally heated to a higher temperature throughout its cross section at the end of the billet that is first drawn through the extruder than the cross section at the opposing end of the billet. This is done since the extrusion process itself is exothermic and heats the billet as it passes through the extruder. If the billet was uniformly heated through its cross section along its entire longitudinal axis, the opposing end of the billet would be overheated prior to extrusion and experience sufficient heat deformation to make extrusion impossible.
One method of achieving gradient induction heating of an electrically conductive billet, such as an aluminum alloy billet along its longitudinal axis, is to surround the billet with discrete sequential solenoidal induction coils. Each coil is connected to an current source at supply line frequency (i.e. 50 or 60 Hertz). Current flowing through each solenoidal coil establishes a longitudinal flux field around the coil that penetrates the billet and inductively heats it. In order to achieve gradient heating along the billet's longitudinal axis, each coil in sequence from one end of the billet to the other generally supplies a smaller magnitude of current (power) to the coil. Silicon controlled rectifiers may be used in series with the induction coil to achieve adjustable currents in the sequence of coils.
Use of supply line frequency makes for a simple current source but limits the range of billet sizes that can be commercially heated in such an arrangement. Penetration depth (in meters) of the induction current is defined by the equation, 503(ρ/μF)1/2, where ρ is the electrical resistively of the billet in Ω·m.; μ is the relative (dimensionless) magnetic permeability of the billet; and F is the frequency of the applied field. The magnetic permeability of a non-magnetic billet, such as aluminum, is 1. Aluminum at 500° C. has an electrical resistivity of 0.087 μΩ·meter. Therefore from the equation, with F equal to 60 Hertz, the penetration depth can be calculated as approximately 19.2 mm, or approximately 0.8-inch. Induction heating of a billet is practically accomplished by a “soaking” process rather than attempting to inductively heat the entire cross section of the billet at once. That is the induced field penetrates a portion of the cross section of the billet, and the induced heat is allowed to radiate (soak) into the center of the billet. Typically an induced field penetration depth of one-fifth of the cross sectional radius of the billet is recognized as an efficient penetration depth. Therefore an aluminum billet with a radius of 4 inches results in the optimal penetration depth of 0.8-inch with 60 Hertz current. Consequently the range of billet sizes that can be efficiently heated by induction with a single frequency is limited.
One objective of the present invention is to provide an apparatus and a method of gradient inductive heating of a billet with a frequency of current that can easily be changed for varying sizes of workpieces.